1. Field of the Invention:
The present invention relates to a rotary electric tool such as a motor-driven screw driver or drill. More particularly, it is concerned with a rotary electric tool equipped with a variable speed gearing using a differential gear mechanism and also equipped with a torque adjusting cam.
2. Prior Art:
Conventional motor-driven screw-drivers and drills generally employ a torque adjusting cam as well as balls and springs as a torque setting structure.
It is the recent desire that the speed of the output shaft be changeable; for example, an automatic two-step speed changing mechanism has recently been desired. And it is necessary that the control for changing speed be made according to increase or decrease of the load exerted on the output shaft relative to a preset torque value.
In order to attain both functions of torque control and speed change using a combined structure of the aforementioned torque adjusting cam and balls, it is necessary that the engaged portion (axial depth) of the balls and the torque adjusting cam be taken large. This is because a speed change signal must be generated before slipping of the torque adjusting cam and to this end it is necessary for the torque adjusting cam to have a corresponding stroke of movement.
Therefore, the ball diameter becomes large for attaining the object in the above-mentioned structure, resulting in that the entire system becomes larger in size (larger in outside diameter).